Guarded Autonomy for Satellites: A Policy-Gated AI Agent Framework for Safe Onboard Fault Response

Spacecraft increasingly face faults that onboard rule-based systems were never designed to handle, yet flight operators cannot give a probabilistic AI agent direct control of a vehicle. This paper presents AEGIS-Sat, a framework that places a deterministic policy gate between an AI agent and the spacecraft. The agent observes a fault and proposes a single structured recovery action; it can execute nothing itself. A policy gate, written as human-readable YAML, rules on every proposal — AUTO, AUTO+LOG, ESCALATE, or DENY — after checking hard safety invariants and a context-adaptive autonomy envelope. Every proposal, ruling, and state snapshot is recorded in a hash-chained, tamper-evident audit ledger. In a fault-injection evaluation (6 scenarios × 3 setups × 100 episodes = 1,800 episodes), an unguarded agent recovered from 88% of episodes but executed 75 unsafe actions; the guarded agent executed zero unsafe actions and recovered in 100% of episodes, at the cost of 659 ground escalations. A rule-only FDIR baseline was equally safe but fully recovered from only 33% of episodes. The gate and benchmark are anchored on real telemetry from ESA's OPS-SAT-AD dataset (detector AUC 0.887).